Crystalline selenium plate



United States Patent CRYSTALLINE SELENIUM PLATE Carl L. Meyer and Paul G. Andrus, Columbus, Ohio,as-

signers, by mesne assignments, to Haloid Xerox Inc.,

Rochester, N. Y., a corporation of New York No Drawing. Application March 14, 1955 Serial No. 511,415

(Filed under Rule 47(a) and 35 U. S. C. 116) 1 Claim. (Cl. 96- 1) This invention relates in general to xerography, and in particular to a new photoconductive insulating or light sensitive plate for xerography.

In the art of xerography, as originally disclosed in Carlson, U. S. Patent 2,297,691, a photosensitive member comprising a photoconductive insulating layer or coating on a conductive backing member or support is electrically charged, exposed to a pattern of light and shadow to be recorded, whereby there is formed an electric charge pattern on the plate surface, and then is utilized for print making by development or deposition of electroscopic particles on the charge pattern. The electric image, known in the art of xerography as an electrostatic latent image, may be utilized by other methods and for other purposes, but according to present commercial operations preferably is utilized by development to form a visible image or print.

According to the Carlson disclosure, the photoconductive insulating layer may comprise anthracene or sulfur, or if desired, a mixture of a major amount of sulfur and a minor amount of selenium. Carlson discloses that each of these is an operable photoconductive insulator but that the anthracene is photographically the fastest member for the preferred member of the class. More recently it has been found that vitreous or amorphous selenium is a more satisfactory photoconductive insulator, inasmuch as its photographic speed is many times faster than that of anthracene, with the result that xerography is capable of commercial operation including projection exposure to light images passed through a lens system.

According to the present invention, it has been found that a xerographic plate can be made by placing a layer of monoclinic selenium crystals on a conductive backing support and forming these crystals into firm physical contact and good electrical contact with the conductive backing. The thus-formed Xerographic plate is characterized by spectral sensitivity differing from that of vitreous or amorphous selenium and, specifically, by substantially increased photon response in the red spectral region. Whereas amorphous selenium appears to be responsive predominantly and almost exclusively in the blue spectral range, the monoclinic selenium layer of the present invention is particularly distinguished therefrom by substantial sensitivity in other spectral regions such as, for example, red.

The new xerographic plates may be made by any desired process reasonably calculated to place the monoclinic selenium crystals in a uniform layer on the conductive surface and to bring the crystals into firm physical contact with the backing member While simultaneously achieving and maintaining good electrical contact between the crystal and the backing surface. Thus, for example, plates have been prepared by sprinkling a layer of the monoclinic selenium crystals on a metal backing plate and pressing this layer into the plate surface with a relatively high physical pressure. If desired, the monoclinic selenium crystals may be applied in the form of a paint 2,862,817 Fatented Dec. 2, 1958 0r lacquer with the selenium crystals being employed together with a suificient quantity of a film forming or binding lacquer base adapted to secure and maintain the crystals as a layer on the plate surface. Similarly, a plate can be prepared by pressing the crystals onto and into the conductive backing surface and by spraying or otherwise coating over the plate surface a small quantity of a lacquer or the like so as to add to the permanency of the plate. It is to be understood that the film forming lacquer or binder which is employed is an insulating material such as to leave the xerographic plate as a good insulator in the absence of activating radiation. Thus, it has been found that a xerogr'aphic plate having a layer composed of photoconductive particles in an insulating hinder or protected by an insulating film forming agent is a good xerographic plate capable of producing fine quality reproductions of a pattern of light and shadow, or other pattern of activating radiation.

It is to be understood that the term activating radiation as employed herein is intended to include both visible light and other light radiation of longer and shorter wave lengths such as infrared radiation and ultraviolet radiation, and to'include also such other radiation as may activate the photoconductive insulator, including but not limited to, X-rays, gamma rays, and the like.

The general nature of the invention having been set forth, the following. example is presented in illustration but not limitation of a specific embodiment of the invention.

Example I Monoclinic selenium crystals were prepared by dissolving selenium in carbon disulfide and then precipitating a part of the selenium by adding to the solution an equal quantity of benzene. The selenium crystals were filtered off and were washed with benzene and dried.

Monoclinic selenium crystals as thus prepared were sprinkled onto a clean brass plate to form a uniform layer, followed by pressing the layer of the crystals onto and into the plate surface at a pressure of 10,000 pounds per square inch. The plates thus produced were examined visually and found to have a layer of the monoclinic selenium crystals of apparently uniform thickness pressed securely onto the brass surface. These plates accepted a positive charge of 200 volts and maintained this charge in the absence of activating radiation for a suflicient time to permit manual operation of the xerographic cycle, which cycle includes charging, exposing, developing with electroscopic material, and transferring the electroscopic material to an adjacent print receiving surface. The plates showed sensitivity to light generally throughout the visible spectrum, being most sensitive in the blue spectral range and having substantial sensitivity in the red spectral range. Spectral sensitivity was compared against plates comprising vitreous or amorphous selenium deposited on a backing surface in vacuo on the one hand and plates printed by pressing flakes of vacuum deposited vitreous or amorphous selenium onto a brass plate surface. In each case the pressed monoclinic selenium plate exhibited a substantially sharper sensitivity peak in the red spectral range than did the plates of vitreous selenium, and compared wi-th.the blue spectral sensitivity showed substantially improved red spectral sensitivity. Plates as thus prepared are considered to be more nearly panchromatic in their sensitivity than are the comparative plates of vitreous or amorphous selenium.

It is specifically to be understood that monoclinic selenium crystals as employed in the preparation of the plate disclosed in Example I may be employed in a mixture with other photoconductive insulator materials. For example, binder-type plates comprising a photoconductive particulate material in an insulating binder may be employed with mixtures of monoclinic selenium crystals with other desired material, such as for example, finely divided mixtures of vitreous or amorphous selenium or other photoconductive materials such as phosphor type materials, including but not limited to various sulfides and oxides of zinc, cadmium, calcium and the like. Likewise, the monoclinic selenium crystals may be pressed onto a conductive surface either alone or in a mixture with other particulate photoconductors andmay be employed in that form or may be coated or bound thereto by an insulating binding material.

In any case, the xerographic plate formed according to the present invention is particularly adapted to the formation of an electrostatic latent image or to the formation of other electrical charge pattern by the combined action of electric field and a pattern of activating light radiation. Most usually, the plate is employed by charging its surface uniformly, for example, by rubbing or by spraying a corona ion discharge thereover, after which it is exposed to light to form an electrostatic charge pattern. The pat-tern may thereafter be utilized by development with electroscopic material and, preferably transferring and affixing the developed image to a print surface, or by other methods as may be desired.

What is claimed is:

A xerographic plate having substantial sensitivity in the red spectral range comprising an electrically conductive backing support and pressed onto and into the surface of said electrically conductive support and in close electrical contact therewith a uniform layer of photoconductive insulating monoclinic selenium crystals firmly imbedded in said support, the plate being capable of accepting and retaining an electric charge in the absence of activating radiation and of dissipating said charge upon exposure to activating radiation including radiation in the visible red spectral range.

References Cited in the file of this patent UNITED STATES PATENTS Middleton Dec. 22, 1953 

